The present invention relates generally to a cigarette with reduced combustion of smoking material during quiescent periods between puffing. The invention also pertains to a cigarette having regulated combustion of smoking material between and during puffing.
A typical cigarette contains 750 mg to 800 mg of tobacco. Approximately 20 mg of this tobacco is burned during a puff, while approximately 50 mg is consumed between puffs. The smoke generated by the burning tobacco during a puff is termed xe2x80x9cmainstream smokexe2x80x9d, while the smoke generated between puffs is termed xe2x80x9csidestream smokexe2x80x9d. Since a large portion of the tobacco is wasted during quiescent periods between puffs, practitioners have attempted to reduce the combustion of tobacco during these quiescent periods.
Commonly assigned U.S. Pat. No. 5,159,940 to Hayward et al. presents one technique for reducing sidestream smoke in a cigarette. As shown in FIG. 1, the cigarette 2 disclosed in this patent consists of a tubular member 4 comprised of plural sections. A first section is comprised of a heat source 10 composed substantially of carbon. The heat source 10 may also contain catalysts or burn additives to promote combustion. The heat source 10 is secured to the tubular member 4 by a retaining member 16, such as metal clips. A second section of the cigarette 2 includes a substrate 14, which comprises tobacco filler mixed with an aerosol precursor, such as glycerine or propylene glycol. A third section of the cigarette 2 comprises an expansion chamber 8. A fourth section comprises a mouthpiece filter 6, such as a cellulose acetate filter.
The above-described cigarette functions in the following manner. A user ignites the heat source 10, upon which the carbonaceous material begins to burn and generate heat. The heat generated by the heat source 10 vaporizes the aerosol precursor in substrate 14 and gases are generated containing flavor extracted from the tobacco in the substrate 14. The gases are drawn into the expansion chamber 8, where the gases expand and cool to form an aerosol 12. The aerosol 12 is drawn out through the filter 6 for delivery to the user. This cigarette thus operates by generating a flavored aerosol rather than burning the tobacco product in a conventional manner. As such, this cigarette generates little or no sidestream smoke while being consumed.
Another cigarette having reduced sidestream smoke is disclosed by U.S. Pat. No. 5,105,835 to Drewett et al. The cigarette disclosed therein also uses a heat source composed of a carbonaceous material. The heat source in this device is inserted within a plug of tobacco and is in contact with the tobacco. A wrapper of low permeability surrounds the plug of tobacco to restrict the amount of oxygen which passes through the wrapper to the underlying tobacco and heat source, thus preventing free smoulder of the tobacco.
The above-described cigarette functions in the following manner. The consumer lights the heat source and the smoking material. During a puff, both the heat source and the smoking material bum to deliver flavor to the consumer. When the user stops puffing, however, insufficient oxygen reaches the tobacco material to sustain its combustion. The tobacco, therefore, stops burning during such quiescent periods. The carbonaceous heat source, on the other hand, has sufficient thermal energy to remain burning. When the user takes another puff on the cigarette, increased oxygen is fed to the heat source, which increases its"" rate of combustion and the amount of heat generated thereby. This increased heat re-ignites the tobacco. Thus, this device reduces sidestream smoke between puffs and also delivers flavor in a conventional manner by burning tobacco.
The use of relatively thick and/or low permeability wrappers or shells in both of the above-described cigarettes generally reduces the influx of oxygen to the interior of the cigarettes. Thus, in the exemplary case of Drewett, despite the use of small perforations in the outer wrapper, this cigarette may non-uniformly burn the tobacco from puff to puff depending on the strength of the puff and other variables.
Accordingly, it is an exemplary object of the present invention to provide a cigarette having reduced sidestream smoke which has more uniform and controllable combustion characteristics.
This and other exemplary objectives are achieved according to the present invention through a cigarette including an ignition element disposed within a plug of tobacco, which, in turn, is disposed within one or more layers of cigarette paper. A perforated wrapper is then wrapped around the cigarette paper layer(s), such that the inner cigarette paper initially blocks the perforations in the outer wrapper and thereby prevents oxygen from reaching the interior of the cigarette. Other materials can be used to block the perforations besides the paper layer(s), such as waxes or films.
In a preferred embodiments the perforated outer wrapper has a permeability selected to provide enough oxygen to the ignition element to sustain its combustion in quiescent periods between puffs, but to provide insufficient oxygen to sustain combustion in the tobacco between puffs. Thus, the tobacco is extinguished (or subject to a reduced rate of combustion) between puffs, thereby eliminating or greatly reducing the amount of sidestream smoke generated by the cigarette. When a user takes a puff on the cigarette after a quiescent period, oxygen is fed to the ignition element, which increases its rate of combustion and temperature. This, in turn, re-ignites the tobacco. To perform in this manner, an ignition element is selected which possesses different thermal characteristics in a low oxygen environment compared to the tobacco. Generally, an ignition element is selected which is less readily extinguished in a low oxygen environment compared to the tobacco. In one exemplary embodiment, the ignition element comprises a carbon element inserted in the plug of tobacco.
During use of the cigarette, the heat generated by the ignition element and tobacco undergoing combustion burns away the paper blocking the perforations, thereby exposing the perforations and creating passageways which allow oxygen to reach the interior of the cigarette through the outer wrapper. The perforations are xe2x80x9copenedxe2x80x9d in successive fashion as the ember of the ignition element advances inward from the distal end of the cigarette. That is, perforations located at the distal end of the cigarette are opened first, followed by perforations located successively further inward from the distal end. In this manner, oxygen is made available to the ignition element even when the burning portion of the ignition element is recessed within the outer wrapper. Initially, however, perforations located inward from the distal end are closed, such that air will not be drawn undesirably through the base of the cigarette. Accordingly, the cigarette of the present invention reduces sidestream smoke while providing uniform and controllable combustion characteristics.